1. Field of the Invention
The present invention relates to a variable displacement pump used as an oil or fluid pressure source for a vehicle power steering system, for example.
2. Description of the Related Art
In the pump for a power steering system, it is desired that fluid pressurexe2x80x94of an amount sufficient to obtain auxiliary steering power corresponding to a steering state during operation of a steering wheelxe2x80x94is applied to a power cylinder of a power steering system. On the other hand, it is unnecessary to apply such pressure when there is no steering, as when the vehicle travels straight.
Moreover, in a power-steering-system pump, it is desired that supplying quantity of pressure fluid at high speed travel is less than at middle or low speed travel and travel stability at high speed straight travel is kept with rigid feeling of steering wheel at high speed travel.
A positive displacement pump driven by an engine of vehicle is generally used as the pump for a power steering system. The positive displacement pump is such that its discharge flow rate increases as the revolution cycle of the engine increases. Therefore, to use the positive displacement pump for a power steering system, a flow control valve is indispensable. The flow control valve controls the discharge flow rate from the pump so as to produce a definite quantity that is independent of the revolution cycle. However, in the positive displacement pump having such a flow control valve, the load to the engine does not decrease even if a part of the pressure fluid is returned to a tank through the flow control valve, and an energy saving effect is not obtained because the horse power required to drive the pump is the same.
To prevent such a disadvantageous condition, variable displacement vane pumps, that decrease discharge flow rate per one revolution of the pump (cc/rev) in proportion to a decrease of revolution cycle, have been proposed by Japanese Patent Laid-Open No. 200883/1994, Japanese Patent Laid-Open No. 243385/1995, and Japanese Patent Laid-Open No. 200239/1996. These variable displacement pumps are so called engine revolution cycle sensitive pumps, and the discharge flow rate of the pump can be decreased when the cam ring is moved in a direction in which the pump capacity of the pump chamber decreases by an amount corresponding to the magnitude of the fluid pressure on the discharge side of the pump when the engine revolution cycle (pump revolution cycle) increases.
Such a variable displacement pump can provide a large auxiliary steering power while stopped, or at low speed travel, because the discharge flow rate of the pump is increased even when the engine revolution cycle is small as when the vehicle is stopped or is traveling at low speed. Since the engine revolution cycle becomes large and discharge flow rate of the pump becomes small at high speed travel of the vehicle, steering having a moderately rigid feeling becomes possible.
Although a discharge quantity that follows engine revolution cycle is obtained at use as oil pressure source of a power steering system in this kind of variable displacement pump, other conditions, for example, the change of a steering state such as speed of the vehicle, the steering speed, the steering angle and etc. has not been considered in the past. Therefore, the following disadvantageous conditions may arise.
Since the conventional variable displacement pump is engine-revolution-cycle sensitive, when the engine revolution cycle becomes highxe2x80x94such as during acceleration, moving up a slope, and coming down a slopexe2x80x94even during low speed travel, the discharge flow rate from the pump decreases. During a steering operation at low speed travel, the necessary flow rate is not maintained in the power steering system because of the very small discharge of the pump. Therefore, there is the possibility that auxiliary steering power will be lost. Because of this problem, flow rate in the conventional pump is not decreased in an attempt to keep the necessary flow rate.
Therefore, the conventional variable displacement pump has a limit on decreasing the discharge flow rate from the pump when the engine revolution cycle increases. Therefore, a sufficient fluid supply and energy savings cannot be obtained.
According to such a variable displacement pump, from the point of saving energy, the designated auxiliary steering power is obtained by supplying fluid of a designated flow rate when steering is need, and by supplying fluid at almost zero or a necessary minimum flow rate when steering is not necessary. For example, when the variable displacement pump is directly driven by the engine of the vehicle, a discharge quantity from the pump is unnecessary when there is no steering, even when the engine revolution cycle is large. Further the horse power necessary to drive the pump is lowered by decreasing the discharge quantity of the pump of this time.
When this kind of variable displacement pump is controlled, it is desirable to carry out the most suitable pump control according to the travel state of the vehicle by judging whether the vehicle stops, travels with low speed, middle speed, or high speed, and whether or not steering is carried out at each travel state. Therefore, it is necessary to determine such travel state and steering state of the vehicle so that pump control suitably is performed. Further, in order to obtain an energy saving effect in the variable displacement pump, the operating state of the pump and the travel state of the vehicle must be determined in order to carry out control of the pump at a designated condition.
The invention is carried out in view of the above-described situation. An object of the invention is to obtain a variable displacement pump wherein pressure loss of a throttle part provided at a discharge side passage is decreased without response delay when operation of the power steering system requires auxiliary steering power, and wherein necessary and sufficient flow rate is supplied. Further, the variable displacement pump can decrease the power consumption required to drive the pump, shows the maximum saving energy effect, and has high reliability.
Another object of the invention is to obtain a variable displacement pump which is an oil pressure pump for a vehicle power steering system and which provides a comfortable steering feeling by control corresponding to a travel condition such as speed of the vehicle, steering speed, etc. Further, the variable displacement pump can better show an energy saving effect by decreasing its discharge flow rate as soon as possible when steering is unnecessary such as when the vehicle travels straight. The discharge quantity of the pump is instantly increased to a necessary quantity when steering is requested, and designated auxiliary steering power is maintained.
To meet such an object, a variable displacement pump according to the invention comprises: a pump body holding a cam ring for movement in a direction whereby pump capacity of a pump chamber increases and decreases, the cam ring forming a first and a second fluid pressure chambers at opposite sides thereof; a spool movable in an axial direction by a difference in fluid pressure between upper and lower stream sides of a metering throttle connected to a discharge side passage of the pump chamber; and a control valve controlling at least fluid pressure in the first fluid pressure chamber; wherein an electronic driving unit applying axial thrust to the spool of the control valve is provided.
According to the invention, by forcibly applying axial thrust to the spool of the control valve by operating the electronic driving unit in response to a condition wherein steering is required for example, the control valve is electrically controlled in addition to the usual fluid pressure control, and the cam ring is instantly moved as a result.
That is, according to the invention, the spool of the control valve is positioned at a designated place in the axial direction by balancing a difference in upper and lower stream sides of the metering throttle, and discharge flow rate of pressure fluid from the pump chamber can be kept minimum. When axial thrust acts on the spool by the electronic driving unit, discharge flow rate of pressure fluid from the pump chamber can be increased to a desired value by connecting the pump suction side to the first fluid pressure chamber, for example, and by connecting fluid pressure of the lower stream side of the metering throttle to the second fluid pressure chamber.
By selectively connecting fluid pressure of the upper stream side of the metering throttle and the pump suction side to the first fluid pressure chamber, and by connecting fluid pressure of the lower stream side of the metering throttle and the pump suction side to the second fluid pressure chamber in the above-mentioned control valve, the difference in fluid pressure between the first and second fluid pressure chambers for moving the cam ring can be made large, and moving and displacement of the cam ring are surely carried out when necessary.
The variable displacement pump according to the invention further comprises an electronic control unit for driving and controlling the electronic driving unit, wherein the electronic control unit has a steering sensor for detecting steering speed of a steering wheel and drives and controls the electronic driving unit according to a signal from the steering sensor.
The variable displacement pump according to the invention further comprises an electronic control unit for driving and controlling the electronic driving unit, wherein the electronic control unit has a steering sensor for detecting steering speed of a steering wheel and a speed sensor detecting travel speed of vehicle, and drives and controls the electronic driving unit according to a signal from each of these sensors.
According to the invention, a variable displacement pump used for a fluid pressure source of a power steering system has a minimum flow rate when a steering operation is unnecessary, as during straight travel of vehicle. When auxiliary steering powerxe2x80x94by the power steering systemxe2x80x94is necessary, it is possible instantly to operate the electronic driving unit corresponding to steering speed, or to steering speed and speed of the vehicle, by the electronic control unit so as to keep sufficient flow rate of the pump discharge side.